Even before that, with her first "death", where she ended up in some kind of brain-damaged coma, she became completely savage after the birth of her children. Some sort of genetically preprogrammed switch that the Plodex have built-in, as I understand it.
After sub-sequent brain-damaged coma and "treatment" by Master of the World, and intense experimentation in order to aggravate and revive her, by Osborn, there wasn't likely that she had many higher brain-functions left.
It's not even very plausible that she would have more than a few garbled and confusing memories of her death. So I'm going to go with Michael here, Namors killing of her was an act of mercy.
Who would like to go on living as something like that?? Not only was her personality gone, but she had even become a very real and lethal threat to those she loved.
I'm going with Michael on this one.
I could almost accept it was an act of mercy if what followed was a funeral/burial conducted with dignity and respect as befits the beloved wife of the Prince of the Seas. Instead what followed was the repeated desecration of her corpse, including dismemberment and weaponizing her disembodied head. After he threw her head at Osborn he smirked about it in a self-congratulatory manner. It turned her murder into a game of one-upmanship with a psycho. No, I'll be angry about that scene forever, and it reduced my opinion of Namor to the point where I can't see him as anything but a villain.